In a plant of that kind it is known to provide a plurality of ports which maintain communication between the charging shaft and the air-intake duct so that air is supplied on various levels to the fuel contained in the charging shaft and the entire fuel charge is thus rendered glowing so that a single incandescent body is created in the charging shaft and may fill the same throughout its height. Under adverse conditions, particularly during the firing of household refuse which constitutes a low-grade fuel, the charging shaft could gradually become clogged in this way so that the operation of the plant may have to be discontinued.